Autoimmunity results from a breakdown in the normal tolerance mechanisms that allow the immune system to properly regulate responses to self-antigens. Progress has been made in the understanding of human autoimmune diseases through the study of mouse model systems. One such system is the X-linked scurfy (sf) mutant, where affected males develop a severe autoimmune lymphoproliferative disease that is fatal by 20-24 days of age. Initial experiments have shown that these symptoms are the result of chronic in vivo T cell activation, with overproduction of a wide variety of cytokines. The gene that is mutated in these mice has been cloned and shown to be a member of the forkhead/winged-helix family, now known as FOXP3. Initial biochemical analysis of Scurfin (the protein encoded by FOXP3) has shown that it is a transcriptional repressor, acting on genes regulated by NF-AT and AP-1. T cells from mice that overexpress this gene fail to proliferate or produce cytokines upon TCR stimulation in vitro, and also fail to respond to immunological challenge in vivo. The human homolog of this gene has also been identified and shown to be mutated in the Immune dysregulation/Polyendocrinopathy/Enteropathy/X-linked (IPEX) syndrome, where affected males develop a variety of autoimmune diseases, including IDDM, and die in their first year. Taken as a whole, these data are consistent with a role for Scurfin in the regulation of T cell responses to antigenic stimulation. Our model proposes that levels of Scurfin in T cells act to set a threshold for functional T cell activation following TCR engagement. Lack of Scurfin in scurfy mice causes a break in tolerance to self antigens due to a lowered threshold for T cell activation. On the other hand, overexpression of Scurfin in T cells raises the threshold for functional activation such that these cells are non-responsive to TCR engagement. However, the mechanism by which Scurfin regulates self-tolerance remains unknown. The experiments in this application are designed to provide insight into the ability of Scurfin to regulate the activation of normal and pathogenic T cells. These analyses will shed light on the regulatory pathways involved in the development of autoimmune disease, and will serve as a model for human immune-system abnormalities. [unreadable] [unreadable]